For use in a microwave hyperthermia therapy against cancers, various thermometers have been used employing an optical fiber for measuring the temperature at local portions of a body. Optical thermometers have been used for the reasons that correct measurement is obtained without interference from electromagnetic waves and no electric shock is given to the living body. It is desired to use the optical thermometer not only for the hyperthermia apparatus but also for extracorporeal blood circulation instruments such as an artificial heart-lung device or an artificial dialysis device, as well as for probing the blood during cardiac catheterization, since it is capable of reducing the danger of electric shock.
At present, there have been proposed optical thermometers employing optical fibers in four principal systems.
A first system is a sensor using a semiconductor as a transducer. That is, the semiconductor usually exhibits an energy gap that varies depending upon a change in the temperature and, hence, exhibits an optical absorption and an accompanying light transmission spectrum that changes thereto. Therefore, there has been proposed an optical fiber sensor by utilizing such properties (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 62-85832). For example, InGaAs and GaAs may be used as semiconductors. FIG. 9(A) shows an example of the constitution of a temperature sensor using such a semiconductor. A sensor 11 composed of the above semiconductor is fixedly provided to an end of optical fiber 10 on the side closer to a body that is to be measured, and a suitable reflector plate 12 is brought into contact with the sensor. Light having a suitable wavelength is permitted to be incident (L.sub.IN)on the other end of the optical fiber: the incident light is reflected by the reflector plate 12 via the semiconductor sensor 11 and returns back to the incident end passing through the semiconductor sensor 11 again. The intensity of light at this moment is measured to determined the temperature of the body that is being measured. FIG. 9(B) shows a relationship between the wavelength of light in the semiconductor and the transmittance factor for a semiconductor sensor having a thickness of 250 .mu.m, from which it will be understood that the transmittance changes depending upon the temperature. (Curve "A" illustrating the relationship at 53.degree. C., and curve "B" illustrating the relationship at 40.degree. C.) By utilizing these characteristics, therefore, it is possible to fabricate a temperature sensor which works depending upon light. However, this method lacks precision for a change in the temperature and has not been put into practical use in medical applications.
A second method is a sensor utilizing a change in the refractive index of a cladding material. According to this system, there is provided a temperature sensor in which, as shown in FIG. 10(A), a cladding 13 is removed from the end of the optical fiber 10, and a cavity 15 containing glycerine 14 therein, is formed at this portion (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 59-160729). The refractive index of the glycerine 14 changes depending upon the temperature and, hence, the angle of reflection of light changes on the interface between the core 16 and the cladding 13. As a result, the intensity of light returning (i.e., reflected by the reflector plate 12) from the end of the fiber changes. Measuring the quantity of light that has returned makes it possible to measure the temperature.
That is, as shown in FIG. 10(B), the DC voltage that is converted through the sensor from the quantity of returned light undergoes a change depending upon a change in the temperature. Therefore, measurement of the DC voltage makes it possible to measure the temperature of the material being measured. In the sensor of this type, however, the end of the probe has insufficient strength. Moreover, it is difficult to fabricate the probe in a small size.
A third system is a sensor which utilizes a change in the color of liquid crystals. According to this system which utilizes a change in the color of liquid crystals depending upon the temperature, there is proposed a sensor obtained by fastening a cavity 15 made of a very narrow glass tube containing liquid crystals 17 at the end of the optical fiber 10 (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 57-63430). FIG. 11 is a diagram illustrating its principle.
FIG. 11 shows a temperature sensor according to the above-mentioned third system, wherein a cavity 15 containing liquid crystals 17 is placed near the material that is to be measured, and an optical fiber 10 is connected to the cavity 15. An example consists of permitting the light to be input at a free end of the optical fiber 10, and measuring the light reflected by the liquid crystals, in order to calculate the temperature of the material that is being measured. That is, the principle is utilized that the color of the liquid crystals change depending upon the temperature, thus the reflection factor of the incident light changes.
According to this system, furthermore, the optical fiber 10 for incident light may be provided separately from the optical fiber 10' for measuring the reflected light. In this case, the optical fibers are used in a bundled form in which both optical fibers are bundled together.
However, this system is costly and has poor resolution. Moreover, if the glass tube is broken, the liquid crystals, which are toxic, adversely affect the living body.
Next, a fourth system is a sensor which utilizes a change in the intensity of a fluorescent material. That is, the wavelength of fluorescence of some fluorescent materials is shifted depending upon the temperature. The temperature sensor of this system utilizes this property. FIGS. 12A, 12B, and 12C illustrate the principle of this system.
FIG. 12(A) shows that the fluorescence spectrum undergoes a change in intensity and wavelength depending upon the temperature of the sensor (here, it is presumed that the temperatures T.sub.1, T.sub.2, and T.sub.3 have a relationship T.sub.1 &lt;T.sub.2 &lt;T.sub.3). Curve "C" represents the spectrum of the excitation light. Curves D, E, and F represent the fluorescence spectrum of the material at temperatures T.sub.1, T.sub.2 and T.sub.3 respectively. The fluorescent material having such properties may be GaAs/AlGaAs produced by Asea Co. or an inorganic fluorescent material produced by Luxtron Co. or Omron Co., and can be used as a temperature sensor.
FIG. 12B further illustrates such a sensor, wherein an optical fiber 10 is connected to a sensor 84 (shown in FIG. 12C) and a measuring device 80. The measuring device 80 comprises a light source 81 (e.g., a light emitting diode), a temperature analyzing portion 82, and a photodiode 83. The sensor 84 comprises layers of GaAs 85 and AlGaAs 86.
However, although these sensors are capable of taking and maintaining high resolution measurements, a problem resides in that the fluorescent materials are expensive.